PROBLEM TO BE SOLVED: To prevent reuse of a treatment device plug without causing destruction.SOLUTION: A substantially cylindrical plug body 29 of the treatment device plug 21 is formed of an elastic material having optical transparency. The plug body 29 is formed with a ferrule insertion hole 29a in which a ferrule 20 is inserted. The inner peripheral surface of the plug body 29 is formed with an engagement claw 35 to be engaged with a flange 25 that is formed around the outer peripheral surface of the ferrule 20. A photo-curing part 31 composed of a photo-curable material is provided between the inner peripheral surface and the outer peripheral surface of the plug body 29. Before curing of the photo-curing part 31, the ferrule insertion hole 29a is expandable and deformable, so that the engagement claw 35 can get through the flange 25 when being inserted/extracted with respect to the ferrule insertion hole 29a. After the curing of the photo-curing part 31, the expansion and the deformation of the ferrule insertion hole 29a is restricted by the photo-curing part 31. The engagement claw 35 cannot get through the flange 25 when the ferrule 20 is inserted/extracted with respect to the ferrule insertion hole 29a so as to prevent the reuse of the treatment device plug 21.
